Dried algae is more concentrated in nutrients. One tablespoon (8 grams) is sufficient to provide most of the nutrient amounts listed above (, , ).

Spirulina and chlorella contain twice as much protein per portion. Unlike other types of algae, they also contain all of the essential amino acids required by the human body. This makes them complete sources of protein (, ).

Some claim that seaweed is a great plant source of vitamin B12, a vitamin naturally found in meat, poultry, eggs and dairy.

However, there's still debate on whether the form of vitamin B12 found in algae is active in humans (, , , , ).

Finally, seaweed is a rich source of antioxidants. It also contains a good amount of sulfated polysaccharides (sPS), which are beneficial plant compounds thought to contribute to seaweed's health benefits (, , , ).

The thyroid plays several important roles in the body, including in the regulation of your metabolism (, ).

Your thyroid requires a good intake of iodine to function properly. Luckily, iodine is readily available in most varieties of seaweed.

Other sources of iodine include seafood, dairy products and iodized salt.

Failure to get enough iodine from the diet can lead to hypothyroidism.

This can create symptoms such as low energy, dry skin, tingling in the hands and feet, forgetfulness, depression and even weight gain (). Adding seaweed to your diet can help you consume sufficient iodine for your thyroid to function optimally ().

The RDI of iodine for adults is 150 micrograms per day. Most people can meet this requirement by eating several servings of seaweed per week.

That said, certain varieties such as kelp, kombu and dulse tend to contain very high amounts of iodine and should not be eaten frequently, or in high amounts.

Others, such as spirulina, contain very little, so don't rely on them as your only source of iodine.

Bottom Line: Seaweed is a great source of iodine, which can help promote proper thyroid function.

Researchers believe this may be due, in part, to seaweed's ability to affect your levels of the weight regulating hormone leptin. Combined with seaweed's high fiber content, this may help decrease hunger and enhance feelings of fullness ().

In addition, fucoidan, a type of sPS found in seaweed, may enhance fat breakdown and prevent its formation (, , ).

Studies in obese participants report that those given a seaweed supplement for 12–16 weeks lost around 3.5 pounds (1.6 kg) more than those given a placebo (, ).

What's more, seaweed is low in calories, but rich in glutamate, an amino acid thought to give it a savory, umami taste ().

Therefore, seaweed snacks may help boost weight loss by providing a satisfying alternative to more calorie-rich snack options.

Bottom Line: Seaweed may boost fat loss by reducing hunger, increasing feelings of fullness and preventing the accumulation of fat. Its savory taste makes it a great low-calorie snack option.

Research shows that these compounds may have the ability to fight viruses such as herpes and HIV by blocking their entry into cells ().

Unfortunately, not many high-quality studies have been done in humans to support these effects.

Two often-cited studies report that taking seaweed supplements may have the ability to reduce symptoms of the herpes virus and increase levels of immune cells in HIV patients (, ).

However, neither of these studies had a placebo group, which makes it difficult to interpret their results.

A more recent study looked at the effects of taking seaweed supplements in HIV-positive women. Those given 5 grams of spirulina per day developed 27% fewer disease-related symptoms, compared to the placebo group ().

However, no differences in immune cell levels were observed over the 12-week study period ().

Additional studies are needed before strong conclusions can be made.

Bottom Line: Seaweed may have some beneficial effects on your immune system. However, more research is needed.

Seaweed may help improve the health of your gut in various ways. For one, it is rich in fiber, which can help prevent constipation and ensure smooth digestion.

It also contains agars, carrageenans and fucoidans, which are thought to act as prebiotics (, ).

Prebiotics are a type of non-digestible fiber that feed the beneficial bacteria in your gut. The more good bacteria you have in your gut, the less space there is for harmful bacteria to thrive.

Accordingly, animal studies show that taking seaweed supplements may improve the amount of healthy bacteria and reduce the amount of harmful bacteria in the gut more effectively than other types of prebiotics (, ).

Researchers also believe that the prebiotics found in seaweed may have certain anti-inflammatory and antibacterial effects.

This may be partly because, when feeding on prebiotics, the bacteria in your gut produce butyrate. This short-chain fatty acid is believed to have anti-inflammatory effects inside the colon ().

In addition, certain prebiotics may have the ability to block harmful bacteria such as H. pylori from sticking to the gut wall. In turn, this may prevent the formation of stomach ulcers (, ).

That said, consuming it regularly or in high amounts may cause some side effects.

It May Contain High Levels of Heavy Metals

Depending on where they're grown, some varieties of seaweed can contain high levels of mercury, cadmium, lead and arsenic.

The US Food and Drug Administration (FDA) regulates the levels of these chemicals and heavy metals in fresh seaweed. However, supplements are not regulated and may contain levels that are detrimental to health ().

A High Intake May Interfere With Kidney Function and Blood Thinners

Certain varieties of seaweed may contain high levels of sodium and potassium, which can be harmful to individuals suffering from kidney disease ().

Seaweed also contains vitamin K, which may interfere with blood-thinning medications. Those taking blood thinners should make sure to check with a doctor before making it a regular part of their diet.

Some Are Very High in Iodine and May Interfere With Thyroid Function

While iodine is necessary for proper thyroid function, getting too much iodine can be harmful (, , ).

Kelp, dulse and kombu are types of seaweed with the tendency to contain very high levels of iodine. For instance, 25 grams of fresh kombu can contain close to 22 times more iodine than the safe daily limit (, ).

Therefore, these varieties should not be consumed too often, nor in large quantities.

Bottom Line: Seaweed is considered safe for most people. Limit your intake if you tend to prefer high-iodine varieties, or if you take blood thinners or have kidney issues.